1. Field of the Invention
The present invention relates a laminate polarizing plate effective to improve viewing angle characteristics of liquid crystal displays, a method of producing the same and a liquid crystal display comprising the same.
2. Description of the Related Art
Liquid crystal displays which have characteristics of low power consumption, low drive voltage, light weight and flat panel, rapidly spread to devices displaying information such as cellular phones, handheld terminals, monitors for computer and televisions. On account of development of liquid crystal cell technologies, liquid crystal displays having various modes are proposed and it is getting to solve the problems of liquid crystal display relating response speed, contrast and narrow viewing angle. The liquid crystal displays, however, are still pointed out on the problem of their narrower viewing angle compared with cathode ray tubes (CRT); hence, various attempts have been done to expand their viewing angle.
As one of liquid crystal displaying methods to improve the viewing angle, for example, Japanese Patent No. 2548979 discloses a vertical-alignment mode nematic type liquid crystal display (VA-LCD). The vertical-alignment mode passes light through liquid crystal layer without changing polarization thereof due to liquid crystal molecules being aligned vertically against substrate in non-driving state. Therefore, by placing linear polarizing plates on and under a liquid crystal panel in a manner of their polarization axes being orthogonal each other, it is achieved to obtain almost complete black indication giving high contrast ratio when being viewed from front side.
However, the vertical-alignment mode liquid crystal displays equipping only polarizing plates to a crystal cell, when viewed from inclined directions, remarkably decreases contrast by light leakage due to deviation of viewing angle to the equipped polarizing plates from 90°, and generating birefringence on rod-like liquid crystal molecules in the cell.
To depress this light leakage, it is necessary to dispose optical compensation films between a liquid crystal cell and linear polarizing plates; for this purpose, conventionally applied methods include the method that each one of biaxially phase retarder films being independently disposed between a liquid crystal cell and, respective upper and lower polarizing plates; the method that each one of an uniaxially phase retarder film and a completely biaxially phase retarder film being independently disposed respectively on and under a liquid crystal cell; or the method that both of an uniaxially phase retarder film and a completely biaxially phase retarder film being co-disposed at one side of a liquid crystal cell. JP-A No. 2001-109009 discloses that, in a vertical-alignment mode liquid crystal display, each of an a-plate (positive uniaxially phase retarder film) and a c-plate (completely biaxially phase retarder film) is independently disposed between a liquid crystal cell, and respective upper and lower polarizing plates.
The positive uniaxially phase retarder film is a film of which ratio R0/R′ of an in-plane retardation value (R0) to a retardation value in a thickness direction (R′) is approximately 2; and the completely biaxially phase retarder film is a film of which in-plane retardation value (R0) is nearly zero. When letting nx to the refractive index of in-plane slow axis of film, ny to the refractive index of in-plane fast axis of film, nz to the refractive index in thickness direction, and d to the film thickness, the in-plane retardation value R0 and the retardation value in a thickness direction R′ are respectively defined by the following formula (I) and (II).R0=(nx−ny)×d  (I)R′=[((nx+ny)/2−nz)×d  (II)
Due to nz≈ny in a positive uniaxially phase retarder film, it results R0/R′≈2. Even in an uniaxially phase retarder film, R0/R′ varies in a range approximately 1.8 to 2.2 due to fluctuation of film elongation conditions. Due to nx≈ny in a completely biaxially phase retarder film, it results R0≈0. Since the completely biaxially phase retarder film is a film of which refractive index is different (or smaller) only in a thickness direction, it has a negative uniaxially phase retardation, and is alternatively called a film having an optical axis in normal line or, as aforementioned, a c-plate. The biaxially phase retarder film attains nx>ny>nz.
Above described methods such that each of a biaxially phase retarder film being independently disposed between a liquid crystal cell and, respective upper and lower polarizing plates, each of an uniaxially phase retarder film and a completely biaxially phase retarder film being independently disposed respectively on and under a liquid crystal cell, or both of an uniaxially phase retarder film and a completely biaxially phase retarder film being co-disposed at one side of a liquid crystal cell, are performed by a complex production procedures or economically disadvantaged.
The polarizing plate is conventionally used in a form of a laminate of a protection layer on one or both sides of the polarizer, and a triacetylcellulose film is generally used as the protection layer; many attempts have been tried to replace the protection layer with other resins or to give a phase retardation properties to the protection layer. For example, JP-A No.H08-43812 discloses that at least one of protection layers of a polarizer is constituted by a birefringent film. As another example, JP-A No.H07-287123 discloses that the protection layer of a polarizer is constituted by norbornene resins (cyclic olefin resins).
As alternation, it is known that a layer exhibiting refractive index anisotropy is formed by coating some kinds of solutions or dispersions. For example, JP-A No.H07-191217 discloses that a coating solution dissolving a discotic liquid crystal in an organic solvent is coated on a transparent support film, followed by aslant aligning and then fixing the liquid crystal to obtain an optically anisotropic element, and the optically anisotropic element is disposed at least one side of a polarizer to form an elliptic polarizing plate. (corresponding to U.S. Pat. No. 6,060,183 JP-A No.H10-104428) discloses that a phase retarder film is formed by a layer containing an organic modified clay composite able to disperse in an organic solvent. WO94/24191 (corresponding to JP-A No.H08-511812) discloses that a polyimide film prepared from a soluble polyimide solution is used as a negative birefringent anisotropic layer for liquid crystal display devices. WO96/11967 (corresponding to JP-A No.H10-508048) discloses that a negative birefringent film prepared from a rigid chain polymer comprising polymers exhibiting a negative birefringent anisotropy such as a polyamide, a polyester, a poly(amide-imide) or a poly(ester-imide) is applied to liquid crystal displays. Moreover, (corresponding to U.S. Pat. No. 5,196,953 JP-A No.H05-249457) discloses that a multi-layered thin film alternately laminated with materials having different refractive index is used as an optical compensation layer for liquid crystal displays.